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Enhancing microplastic analysis: Hyperpolarized ¹²⁹xe NMR to probe surface morphologies and pore structures
Summary
Researchers applied hyperpolarized xenon-129 NMR spectroscopy to probe the surface morphology of microplastic particles, detecting surface roughness and cracks not visible by standard techniques. Surface characteristics revealed by NMR influence how microplastics interact with environmental molecules and biological surfaces.
Microplastic particles (MPP) exhibit diverse behaviors influenced by their surface morphology. The drag coefficient and, thus, transport mechanisms depend on the surface area, roughness, and shape.[1] Together with cracks, they affect the interaction between the polymer and the molecules from the environment, potentially accelerating ageing through freeze-thaw cycles. Additionally, the build-up of an eco-corona depends on the surface morphology, further impacting toxicity and environmental effects. Thus, understanding MPP surface morphologies is paramount. Current adsorption measurements, while informative, only capture surface area changes over weathering periods. For example, polystyrene particles experience a surface increase from 0.5 to 2.5 m²/g and size reduction from 160 to 20 µm after 3200 h of weathering. Although highly sensitive, Atomic Force Microscopy (AFM) struggles to probe deep cracks and requires extensive measurements to quantify features in each sample. These limitations highlight the necessity for a more sensitive and comprehensive method for surface analysis. In this regard, Hyperpolarized (HP) ¹²⁹Xe Nuclear Magnetic Resonance (NMR) is a successful technique. The chemical shift is very sensitive to its confinement, making it an ideal probe to detect changes in porosity within the nanometer size range. With our home-built Polariser, we create hyperpolarised xenon gas, increasing sensitivity by five orders of magnitude and, therefore, rapidly detecting mesopores and small amounts of micropores.[2] Utilising ¹²⁹Xe NMR measurements, the development of multiscalar pore structures in PS MPPs postulated by Meides et al. is verified.[3] Referencing with samples of known pore sizes enables the relation function between pore size and chemical shift, revealing a significant increase in micropores by a factor of four in volume. Thus, HP ¹²⁹Xe NMR provides a deeper understanding of microplastic surface characteristics, improving our ability to assess their environmental impact and toxicity. [1]Kabir et al.,PolymJ,2015,47,564 [2]Stäglich et al., J.Phys.Chem.A.,2022,126,16,2578-2589. [3] Meides et al., Environ.Sci.Technol.,2021,55,7930 Also see: https://micro2024.sciencesconf.org/559663/document